The present invention relates to circuits for controlling the power supplied to a light source to maintain the intensity of illumination in a given location at a predetermined level. More specifically, the invention relates to a circuit for compensating for variable reflectance of an illuminated area.
As is well known, different tasks to be undertaken in different working environments require different illumination levels. If illumination levels fall below the desired level, the safety and efficiency of personnel suffers. If illumination levels are raises significantly above these desired levels, energy usage increases. In fact, many municipalities have law which prohibit over-illumination of work areas in order to conserve valuable energy resources.
Prior to the present invention, there were three known techniques available for adjusting light levels to provide adequate, but not superfluous lighting. The first of these approaches involves the installation of lighting which is certain to be adequate enough to meet the minimum lighting level, the subsequent measurement of the incident illumination in the lighted area through the placement of a photosensitive device at the working surface, and the subsequent adjustment of the power supplied to the lighting element while monitoring the photosensitive device to provide the correct lighting level.
More often, a compromise is merely made in the selection of lighting fixtures in a room which is sure to provide adequate lighting, a practice which often over-lights an area and wastes energy.
Even in those installations where a careful adjustment is made of the lighting level, using a photometer, the lighting level must be set assuming no natural light is available, so that the work space is usable on cloudy days or during times of darkness. Under such circumstances, when natural light adds to the illumination level, energy is often wasted.
The second obvious technique for maintaining proper incident illumination levels is the placement of a photosensitive device at the work surface, directed at the source of lighting. Such a photosensor can be used to control the power supplied to the lighting source and will adequately compensate for changes in natural illumination levels. The installation of such a system, however, involves added expense in terms of wiring remote photosensors to the source of light. In addition, the photosensors are necessarily located where they can be inadvertently covered by items on the work surface or can become dirty, both situations leading to a false indication of inadequate lighting and a resultant over-lighting of the work area.
An additional prior art technique is the placement of a photocell, or other light sensitive device, on a light fixture, directed toward the area to be illuminated. Such a device will, of course, control the light source and compensate for changes in the natural illumination level within the lighted area. Once installed in a work area, these devices can be adjusted so that a fairly constant level of illumination can be provided. However, if the reflectance of the work area is altered, a false indication of illumination falling on the work surface will be provided by the system and the lights will be improperly adjusted. Thus, for example, if lights are installed with such a photosensor, in a work area which includes relatively dark furniture and floors, and the furniture or floors are later changed to a lighter color, or even in the instance where a large sheet of paper is placed on a desk, these changes in work area reflectance will be interpreted as an increase in the illumination level, even though no such increase has occurred. The system will thus respond by lowering the illumination level, possibly below that which is satisfactory for the work to be performed. Alternatively, if the lights are initially adjusted for a work area which is fairly light or reflective, and furnishings are later added which have a low reflectance, the lighting will be automatically increased, resulting in unnecessarily high levels of illumination.